The present invention relates to systems employed for controlling the emission of vapors from a vehicle fuel tank and particularly such systems as employed on light trucks and passenger cars.
Mandated regulations for controlling vehicle on-board fuel vapor emission require that the vapor from the fuel tank vent system be stored during periods of engine in operation; and, it is thus necessary to purge the stored vapors during engine operation in order for the vapor emission control system to continue to function for its intended purpose. It is thus necessary to maintain the integrity of the components of the emission control system over the service life of the vehicle and to prevent leakage of fuel vapor from the system.
Heretofore, a known system for controlling fuel tank vapor emission, as shown in FIG. 2, has included a storage device 1 connected to receive fuel vapor directly through a conduit from the fuel tank 2, with the storage device or canister 1 having an atmospheric purge inlet 3 which may be opened or closed by a control valve 4 connected to an electronic controller 5. The fuel tank has a tank pressure sensor 6 connected to sense the internal pressure in the tank which sensor provides an electrical input to the electronic control unit 5.
The canister has a vapor outlet line 7 connected to provide vapor flow through a control orifice 8 with the downstream side of orifice 8 connected to the inlet of a flow control valve 9 which has its outlet connected to the combustion air inlet or intake manifold of an engine 10. Flow control valve 9 is an electrically operated valve controlled by the electronic control unit 5.
An electrically operated normally open valve 11 is connected to bypass the orifice 8 and is also controlled by the electronic control unit 5. A differential pressure sensor 12 is connected to sense the pressure drop across the orifice 8; and, pressure sensor 12 provides an electrical input signal to the electronic controller 5.
In operation, for performing leak tests, it is necessary for the controller 5 to close the atmospheric vent valve 4 and the bypass valve 11 before any leak tests can be performed. The flow control valve 9 is then modulated to provide a predetermined negative gauge pressure or vacuum in the tank, as sensed by the tank pressure sensor 6; and, the pressure sensed by the differential pressure sensor 12 is read. The flow through orifice 8 may then be determined from lookup tables and the flow rate compared with a threshold value to determine if the flow is in excess of the threshold and therefore that leakage exists.
The above-described known system thus requires not only a vapor management valve in the form of flow control valve 9 but also a separate electrically controlled bypass valve 11 which must be actuated in order to perform the leakage test. This arrangement has thus been costly to implement in high volume production of light motor vehicles and it has therefore been desired to provide a simpler more cost effective way of providing a fuel tank vapor emission system and leak testing same.
The present invention provides a low cost, simple and reliable system and method for performing a diagnostic leak test on a fuel tank vapor emission control system as employed in motor vehicles. The arrangement of the present invention provides a pressure relief valve having a bleed orifice formed therein which is disposed in the line from the vapor storage canister to the electric valve controlling flow of fuel vapor to the engine combustion air inlet,. A differential pressure sensor is disposed to sense the pressure drop across the pressure relief valve when closed; and, a tank pressure sensor provides tank pressure data for the test. An electronic controller closes the atmospheric air inlet valve to the canister, modulates the vapor flow to the engine air inlet to provide a desired tank pressure for conducting the test. The differential pressure sensor then provides pressure data to the controller from which flow to the bleed orifice may be determined by comparison with differential pressure values for known flows from a lookup table. The computed flow may then be compared with a threshold value to determine whether leakage exists.
The present invention thus provides a unique and novel leak test system which eliminates the need for a separate electrically operated bypass control valve for the flow measuring orifice.